National Wildlife Refuge
961 E Minidoka Dam Rd.
Rupert, ID 83350 - 9414
Phone Number: 208-436-3589
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Continued . . . Southeastern Idaho is one of the more geologically interesting areas in the country.
The black volcanic rock you see everywhere is basalt, the legacy of the Yellowstone Hot Spot, a geologic phenomenon deep under the earth's crust. It causes extremely large and violent eruptions of rhyolite every few million years, interspersed with the type of activity you see at Yellowstone National Park today.
The Hot Spot remains stationary while the earth's crust moves over it. As the crust moves away, the ground subsides and the explosive volcanism is replaced by oozing basalt flows. The Hot Spot was once under southwestern Idaho, and the scar of the earth's path over the Hot Spot is visible today as the Snake River Plain.
Geologically speaking, the basalt eruptions are still going on. The last one was about 3,000 yeas ago at Craters of the Moon National Monument north of the refuge.
Starting about 17 million years ago, the entire Intermountain West has been stretching apart, creating a series of narrow mountain ranges interspersed with narrow valleys. Geologically, this area is known as the Basin and Range Province. The spreading stopped drainage to the sea and formed land-locked Lake Bonneville, now known as Great Salt Lake. You can see this Basin and Range topography in the mountains and valleys north and south of the refuge.
Northeast of the refuge is a unique feature known as the Great Rift, where crustal spreading created cracks in the basalt flows several hundred years ago. The Great Rift is part of Craters of the Moon National Monument.
About 14,500 years ago, Lake Bonneville overflowed a natural earthen dam at Red Rock Pass near Preston, Idaho. The lake covered some 25,000 square miles, including most of Utah, eastern Nevada, and part of southern Idaho. The flood dropped the lake level by some 350 feet. Geologists have estimated that the flow was almost 60 times that of the Amazon River and lasted for about 2 months.
You can see signs of the flood throughtout the Great Basin, from old shorelines on mountain sides around Great Salt Lake, to boulder deposits and scablands all along the Snake River, to dry waterfalls and canyons just upstream from the refuge, to wider river valleys near Pocatello and to the waterfalls and deep canyons in the Snake River near Twin Falls, Idaho. Near the refuge, the water spread over the plain and the river was miles wide.
Most emigrants considered the Snake River corridor an inhospitable place, hot and dry. To them, it was a place to get through, not a place to settle. To Native Americans, it was home and an area that they treasured.
The North Alternate Branch of the Oregon Trail ran across the north side of the refuge. Although some stretches of the trail are now under water, ruts from the wagons can still be seen in many places.
The main trail ran south of the Snake River and branched into the California and Oregon Trails in the Raft River Valley about 1 mile south of the refuge. Look for the white Oregon Trail markers.
From the late 1800s to the early 1900s, most of the river from Raft River downstream to Milner Dam was sluiced and dredged by miners seeking placer gold. There was a gold mill below the dam just below the rapids.
The islands you see below Minidoka Dam today are spoil banks from the dredging. When the water is low, the remnants of an old dredge can still be seen below Montgomery Bridge.
The Minidoka Dam and Powerhouse are on the National Register of Historic Places. During construction of the dam, there was a small village in what is now Walcott State Park and another across the river.
From 1935 to 1942, there was a Civilian Conservation Corps camp, Camp Minidoka BR-27, just west of the refuge office. The office and stone walls in the park are all that remains of the camp that once had 18 buildings and held 200 men.
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